1. Relation to Other Applications
This application is related to application Ser. No. 07/971225, filed concurrently herewith and assigned to the assignee of this invention.
2. Field of the Invention
Disclosed is an acoustic isolator for use with an acoustic downhole logging sonde that is used for measuring certain rock parameters indicative of selected properties of the sidewall material of a borehole. The isolator inhibits sonic crossfeed between a transmitter and a receiver that are mounted at opposite ends of the sonde.
3. Discussion of Related Art
Acoustic logging tools for measuring properties of the sidewall material of both cased and uncased boreholes are well known. Essentially such tools measure the travel time of an acoustic pulse propagating through the sidewall material over a known distance. In some studies, the amplitude and frequency of the acoustic pulse, after passage through the earth, are of interest.
In its simplest form, an acoustic logger consists of one or more transmitter transducers that periodically emit an acoustic pulse into the formation around the borehole. One or more receiver transducers, spaced apart by a known distance from the transmitter, hears the pulse after passage through the surrounding formation. The difference in time between pulse transmission and pulse reception divided into the distance between the transducers is the formation velocity. If the transducers do not contact the borehole sidewall, allowance must be made for time delays through the borehole fluid.
Throughout this disclosure, the term "velocity", unless otherwise qualified, shall be taken to means the velocity of propagation of an acoustic wavefield through an elastic medium. The term does not mean the velocity of motion of a medium.
Acoustic wavefields propagate through elastic media in different modes. The modes include: Compressional or P-waves, wherein particle motion is in the direction of wave travel; transverse shear or S-waves, which, assuming a homogeneous, isotropic medium, may be polarized in two orthogonal directions, with motion perpendicular to the direction of wave travel; Stonley waves, which are guided waves that progagate along the fluid-solid boundary of the borehole; and compressional waves that propagate through the borehole fluid itself. There also exist asymmetrical flexural waves as will be discussed later.
P-waves propagate through both fluids and solids. Shear waves cannot exist in a fluid. Compressional waves propagating through the borehole fluid may be mode-converted to shear waves in the borehole sidewall material by refraction provided the shear-wave velocity of the medium is greater than the compressional-wave velocity of the borehole fluids. If that is not true, then shear waves in the sidewall material can be generated only by direct excitation.
Among other parameters, the various modes of propagation are distinguishable by their relative velocities. The velocity of compressional and shear waves is a function of the elastic constants and the density of the medium through which the waves travel. The S-wave velocity is, for practical purposes, about half that of P-waves. Stonley waves may be somewhat slower than S-waves. Compressional wavefields propagating through the borehole fluid are usually slower than formational shear waves but for boreholes drilled into certain types of soft formations, the borehole fluid velocity may be greater than the sidewall formation S-wave velocity. The velocity of flexural waves is said to approach the S-wave velocity as an inverse function of the acoustic excitation frequency. Some authors refer to flexural waves as pseudoRaleigh waves.
In borehole logging, a study of the different acoustic propagation modes provides diagnostic information about the elastic constants of the formation, rock texture, fluid content, permeability, rock fracturing, the goodness of a cement bond to the well casing and other data. Typically, the output display from an acoustic logging tool takes the form of time-scale recordings of the wave train as seen at many different depth levels in the borehole, each wave train including many overlapping events that represent all of the wavefield propagation modes. For quantitative analysis, it is necessary to isolate the respective wavefield modes. S-waves are of particular interest. But because the S-wave arrival time is later than the P-wave arrival time, the S-wave event often is contaminated by later cycles of the P-wave and by interference from other late-arriving events. Therefore, known logging tools are designed to suppress undesired wavefields either by judicious design of the hardware or by post-processing using suitable software.
J. Zemaneys, in U.S. Pat. No. 4,516,228, issued May 7, 1985 provides a borehole logging system that employs a compressional wave transmitter and a direct-excitation shear wave transmitter. The transmitters are alternately fired to impart compressional and shear waves in the surrounding borehole formations. A single bender-bar receiver, spaced apart from the transmitter in the borehole is alternately gated so that the voltages across its pair of piezoelectric planar surfaces are subtracted during the expected period of compressional wave output and added during the expected arrival time period of asymmetrical motion of the receiver to provide shear wave output.
As is well known, the acoustic transmitter and the acoustic receivers are mounted at opposite ends of a logging sonde. The body of the sonde is usually of a suitable metal such as stainless steel or the like which is acoustically conductive. Therefore, in order to prevent unwanted acoustic energy traveling up the sonde from interfering with desired acoustic energy propagating through the formation, is it required that an acoustic isolator be inserted in the sonde between the transmitter and the receivers.
U.S. Pat. No. 3,288,245, issued Nov. 29, 1966 to T. O. Anderson, describes a rigid acoustic isolator for a logging tool that employs a liquid/solid interface whose acoustic mismatch provides the desired acoustic isolation between the transmitter and the receiver modules of the logging tool.
U.S. Pat. No. 4,872,526, issued Oct. 10, 1989 to A. Wignall et al. for a Sonic Well Logging Tool Longitudinal Wave Attenuator that consists of first and second outer housings and an inner housing disposed within the first and second outer housings. A first attenuation member is disposed between the first outer housing and the inner housing and a second attenuation member is disposed between the second outer housing and the inner housing. Each of the attenuation members consists of a plurality of rubber washers interleaved with a plurality of steel washers. The attenuation members are fitted between a flange of the respective outer housings and a flange on each end of the inner housing.
There is a need for an acoustic isolator that will be sufficiently flexible to pass through deviated boreholes yet sufficiently rigid to avoid buckling upon encountering an obstruction in the hole as well as to provide axial rotational stability for the modules that make up the sonde as a whole.